The role of oxygen and light in guiding “photoaccumulation” in the Paramecium bursaria‐Chlorella symbiosis

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Abstract
Groups of Paramecium bursaria, containing the symbiotic alga Chlorella, aggregate in a “light trap” – a spot of light in a dark field. Cell populations conform to the shape of the light trap because (1) individual cells in the light either move slowly or not at all; and (2) individuals also, upon leaving the spot of light, undergo a short avoiding reaction – a step‐down response – changing the direction of their forward movement back toward the light. Upon removing the light source, cells become more motile and disperse from the area of the light trap. A similar result is produced in the light by the addition of 10−6 molar 3‐(3,4‐dichloropheny1)‐1,1‐dimethyl urea, (DCMU), which completely inhibits the photosynthetic evolution of O2 by Chlorella‐bearing P. bursaria. However, in the presence of DCMU the step‐down response is not affected. We conclude that P. bursaria accumulates in light through two mechanisms: an “aeroreceptor” senses changes in concentrations of O2 and a “photoreceptor” senses decrease in light intensity. Additionally, methods of quantifying photobehavior have been developed and have been used to show that blue light is a more effective inducer of step‐down photobehavior than is red light. Further studies are underway to examine the roles of ions and of the symbiont in mediating the response of P. bursaria to light.